Influence of Boundary Layer Mixing Mechanisms on the Simulation of Typhoon Wutip:A Direct Hit on the Xisha Islands in 2013

In this study,we simulated the tropical cyclone(TC)Wutip,which originated in the South China Sea in 2013,using three planetary boundary layer(PBL)parameterized schemes within the Weather Research and Forecasting model,i.e.,Medium-Range Forecast(MRF),Yonsei University scheme(YSU),and Asymmetric Convective Model Version 2(ACM2),with different vertical mixing mechanisms.We investigated the effects of different PBL mixing mechanisms on the simulation of TC track,intensity,structure,and precipitation.The results reveal that the surface flux and vertical mixing of PBL jointly influenced the TC throughout its lifecycle,and the simulated TC intensity was closely correlated with the eyewall structure.These three schemes were all first-order and nonlocal closure schemes.However,the MRF scheme was over-mixed,which led to a relatively dryer and warmer near-surface layer,a wetter and colder upper PBL,and thus a simulated eyewall with the smallest wet static energy and weaker convection.Moreover,the MRF scheme produced the smallest 10-m wind speed,which was closest to the observation,and the weakest TC warm-core structure and intensity.The YSU scheme was similar to the MRF scheme,yet it distinguished itself by incorporating an explicit treatment of the entrainment process at the top of the PBL and developing thermal-free convection above the PBL of the eyewall,which significantly increased the wet static energy over the TC eyewall.Thus,the simulated eyewall was more contracted and steeper with stronger upward motion while the eye area became even warmer,finally leading to the strongest TC.The precipitation distribution simulated by the YSU scheme was the most consistent with the observation.The ACM2 scheme used the nonlocal upward and local downward mixed asymmetric convection modes,which reduced the excessive de-velopment of thermal-free convection at the eyewall,and avoided restricting the dynamically forced turbulent motion outside the eyewall,leading to a larger radius of the maximum wind speed,and thus more reasonable structural char-acteristics of PBL and TC intensity.In summary,compared with the YSU scheme and the MRF scheme,the ACM2 scheme demonstrated superior performance in capturing the structure,track,and intensity of Typhoon Wutip.It is important to note that this analysis was based on a specific case study,which might have inherent limitations due to its modest focus.